His73, Often Methylated, Is an Important Structural Determinant for Actin

His(73), has been proposed to regulate the release of P(i) from the interior of actin following polymerization-dependent hydrolysis of bound ATP. Although it is a 3-methylhistidine in the vast majority of actins, His(73) is unmethylated in S. cerevisiae actin. We mutated His(73) in yeast actin to Arg, Lys, Ala, Gln, and Glu and detected no altered phenotypes associated with the mutations in vivo. However, they significantly affect actin function in vitro. Substitution of the more basic residues resulted in enhanced thermal stability, decreased rate of nucleotide exchange, and decreased susceptibility to controlled proteolysis relative to wild-type actin. The opposite effects are observed with the neutral and anionic substitutions. All mutations reduced the rate of polymerization. Molecular dynamics simulations predict a new conformation for the His(73) imidazole in the absence of a methyl group. It also predicts that Arg(73) tightens and stabilizes the actin and that Glu(73) causes a rearrangement of the bottom of actin's interdomain cleft leading possibly to our observed destabilization of actin. Considering the exterior location of His(73), this work indicates a surprisingly important role for the residue as a major structural determinant of actin and provides a clue to the impact caused by methylation of His(73).